Reducing the Risk of Prussic Acid Poisoning
Warm-season annuals can serve as a means to provide forage for livestock during the summer months when the growth of cool-season perennials is slowed. In many cases, regrowth can allow for more than one grazing or harvest to be obtained from these forages. Under certain conditions, there is potential for prussic acid accumulation in some of these warm-season annual species.
What is prussic acid?
Sorghum species like sorghum, sudangrass, sorghum- sudangrass hybrids, and johnsongrass, contain a cyanogenic compound called dhurrin. Under normal circumstances, the dhurrin is bound and non-toxic; however, if the plant tissue is injured by some sort of stressor, the plant cells can become damaged and an enzyme called emulsion can break down the dhurrin, resulting in the formation of a highly toxic hydrogen cyanide compound commonly referred to as prussic acid. Prussic acid concentrations as low as 0.1% of dry tissue is considered dangerous. Prussic acid hinders the animal’s ability to transfer oxygen in the bloodstream. When a lethal dose is consumed, animals die from asphyxiation within minutes. Common symptoms prior to death include excessive salivation, difficulty breathing, staggering, convulsions, and collapsing.
What causes prussic acid accumulation?
The greatest levels of prussic acid can be found in the leafier parts of the plant, particularly in new growth, and young growing plants contain more prussic acid than older plants. Any stress condition or injury can lead to an appreciable accumulation of prussic acid within the plant. Frost damage is one of the more common causes leading to an accumulation of prussic acid, as prussic acid is released very quickly in frozen leaves. However, other stress conditions, such as a prolonged drought or soil nutrient imbalances from excessive nitrogen fertilization and deficient phosphorus or potassium levels can also cause high levels of prussic acid accumulation.
How can prussic acid poisoning be avoided?
Most problems with prussic acid can be avoided with proper management. To reduce the potential of prussic acid poisoning, utilize the following guidelines:
- Select a variety that has a lower prussic acid potential.
- Maintain proper soil fertility and avoid excessive nitrogen.
- Do not graze on nights when frost is likely, as high levels of the toxin are produced within hours after a frost.
- After a killing frost, wait at least 7 to 10 days before grazing or green chopping forage, as prussic acid does not begin to decline until after the leaves have dried.
- After a non-killing frost, do not graze until the regrowth has reached a minimum of 2 feet in height or 2 weeks have passed, as the regrowth will likely contain high levels of prussic acid.
- Utilize heavy stocking rates and rotational grazing to reduce the risk of animals selectively consuming only young growth that is high in prussic acid.
- Test questionable forage to ensure safety before grazing or feeding.
What about harvested forages?
Proper field curing or ensiling can help reduce the potential for toxicity in harvested forages because prussic acid is volatile and some of the toxic components will dissipate as a gas during the drying or fermentation process. Forages should be ensiled for at least 3 to 4 weeks before feeding, or a minimum of 8 weeks if there was a risk of high prussic acid levels at the time of chopping. The prussic acid content in hay can be reduced by as much as 75% during the curing process, so dry hay is typically not hazardous.
Can prussic acid levels be tested?
Forages can be analyzed prior to feeding to ensure the toxic compounds have been reduced to a safe level for consumption. Because the greatest prussic acid concentrations are found in new, leafy growth and livestock generally consume leaves before stems, samples taken for prussic acid analysis should be largely comprised of leaves. Samples should be collected from several places throughout the field and kept in a sealed plastic bag to prevent the volatilization of prussic acid from the sample. Samples should be kept refrigerated and delivered to a testing laboratory as soon as possible.
This article appears in October 2024, Volume 15, Issue 7 of the Agronomy News.